Mining-induced Seismic Events Research Articles

Limitations of Upper Protective Layers as Pressure Relief Measures for Extra-Thick Coal Seam Mining: Insights from a Case Study

Upper protective layer (UPL) mining is extensively utilised as a pressure relief strategy to prevent outbursts and coal bursts. However, when the excavation height of the protected layer is substantial, the depressurisation efficacy of the protective layer may be diminished. This paper takes the Haishiwan coal mine in China as a case study and explores the stress evolution and influencing factors in the mining of extra-thick coal seam beneath the protective layer through theoretical analysis, numerical simulation, and field observation. The results indicate that increasing the excavation height of the coal seam will lead to the upward development of the collapse zone in the overburden of the goaf, with the “masonry beam” structure formed at a higher position by key strata blocks. The overburden above the masonry beam will be supported by the coal rock masse on both sides of the structure, leading to increased stress on the coal seam near the goaf and eliminating the depressurisation effect of the protective layer. Numerical simulation shows that factors such as faults, protective layers, interlayer spacing, and the height of coal seam excavation significantly affect the stress distribution in the protected layer. With the increase in interlayer spacing and the thickness of coal seam extraction, the stress reduction phenomenon of the UPL gradually decreases, especially with an abnormal stress concentration of the gob-side coal seam. Observations of Surface subsidence and the distribution of mining-induced seismic events corroborate the conclusions of theoretical analysis and numerical simulations. The results offer valuable guidance for the mining of extra-thick coal seams and the selection of the UPL.

A Combined Method of Seismic Monitoring and Transient Electromagnetic Detection for the Evaluation of Hydraulic Fracturing Effect in Coal Burst Prevention.

In order to mitigate the risk of roof-dominated coal burst in underground coal mining, horizontal long borehole staged hydraulic fracturing technology has been prevailingly employed to facilitate the weakening treatment of the hard roof in advance. Such weakening effect, however, can hardly be evaluated, which leads to a lack of a basis in which to design the schemes and parameters of hydraulic fracturing. In this study, a combined underground-ground integrated microseismic monitoring and transient electromagnetic detection method was utilized to carry out simultaneous evaluations of the seismic responses to each staged fracturing and the apparent resistivity changes before and after all finished fracturing. On this basis, the comparable and applicable fracturing effects on coal burst prevention were evaluated and validated by the distribution of microseismic events and their energy magnitude during the mining process. Results show that the observed mining-induced seismic events are consistent with the evaluation results obtained from the combined seismic-electromagnetic detection method. However, there is a limited reduction effect on resistivity near the fractured section that induces far-field seismic events. Mining-induced seismic events are concentrated primarily within specific areas, while microseismic events in the fractured area exhibit high frequency but low energy overall. This study validates the rationality of combined seismic-electromagnetic detection results and provides valuable insights for optimizing fracturing construction schemes as well as comprehensively evaluating outcomes associated with underground directional long borehole staged hydraulic fracturing.

Induced seismic activity and source parameter characteristics in the Datong coal mine, China

To clarify the activity patterns and source characteristics of coal mining–induced microseismicity, this study analyzed the spatial distribution characteristics of microseismic events in the Datong coal mining area based on records from the regional digital seismic network. We conducted a detailed characterization of the depth distribution characteristics of microseismic events using the double-difference localization method. Additionally, the source parameters, including corner frequency ( fc), source rupture radius ( r), seismic moment ( M0), source radiated energy ( Es), and stress drop (Δσ), were calculated for 136 mine-induced earthquakes with magnitudes ranging from ML1.3 to ML3.2. The results show that ML ≥ 2.0 mining-induced seismic events occur mainly within numerous microfractures in the Datong mining area. The depth of the seismic sources in the mining area is concentrated at 200∼500 m, with significant north–south differences and a close correlation with the mining depth. The displacement spectra of microseismic sources show agreement with the Brune source model [Formula: see text] attenuation pattern. As M0 gradually increases, r, Δσ, and Es show an increasing trend, while fc gradually decreases, exhibiting characteristics similar to those of tectonic earthquakes. Compared to tectonic earthquakes, coal mining-induced earthquakes have lower corner frequencies and stress drop levels mainly because mining activities alter the originally stable geological structure and stress state, leading to weakened rock strength, decreased elastic modulus, and shallower source depths. These factors contribute to the reduction in corner frequencies. As mining operations continue, microfracturing occurs in the coal and surrounding rock mass, intensifying the dynamic instability of the rock mass that was already under high stress conditions. This situation triggers larger-magnitude, mining-induced seismic events under lower stress conditions.

Physically motivated moment-tensor decomposition for mining-induced seismicity

SUMMARY Compared to existing schemes, the decomposition of moment tensors for mining-induced seismic events into closing-crack and double-couple components has the advantage that each can be interpreted in terms of a physical source process (excavation convergence and slip/shear, respectively). Obviously, not every moment tensor permits such a decomposition, and we translate existing bounds on those that do to the Hudson source-type plot. For moment tensors falling within these bounds, it has previously been noted that there will be a infinite set of possible decompositions in general. We derive an implicit equation defining this set and suggest physically motivated criteria that can be used to select a single decomposition from it. Furthermore, for moment tensors falling outside the source-type bounds, we present a simple geometric scheme for determining the closest moment tensor within them (which can then be decomposed). To demonstrate the methods developed in this paper, we apply them to two catalogues of mining-induced seismicity.

Influence of Vibration and Ground Deformation on Historic Structures: Case Study

When discussing historic structures, a significant problem is how to preserve and protect these structures. The presented paper deals with some issues of the unfavourable impact of industrial activity on some historic structures. Long-term influences from the industrial activity are one of the common sources of damage observed in these structures. Due to the size and volume of such structures, they are characterized by very low resistance to vibration and ground deformations. At the same time, the high cultural and material value of historic structures necessitates their costly protection and repair. Knowing the origin of problems, one may take proper actions to protect them. This paper discusses two important types of impact typical for industrial areas, especially affected by mining, seismically induced vibrations and continuous ground deformations from underground extraction. The presented discussion is based on the case study examples of historical sites located in some industrial areas in the Czech Republic and Poland. They point to different sources of damage to those structures that may arise as well as a combined effect of mining-induced seismic events and land subsidence. The medieval Jeroným Mine represents an underground structure loaded with natural and technical seismicity. The paper also includes a short overview of the process of seismic loading evaluation and basic information about historic structures with respect to seismic standards and land surface subsidence caused by underground mining.

Quantitative assessment of the spatio-temporal correlations of seismic events induced by longwall coal mining

Rock failure process as a natural response to mining activities is associated with seismic events, which can pose a potential hazard to mine operators, equipment and infrastructures. Mining-induced seismicity has been found to be internally correlated in both time and space domains as a result of rock fracturing during progressive mining activities. Understanding the spatio-temporal (ST) correlation of mining-induced seismic events is an essential step to use seismic data for further analysis, such as rockburst prediction and caving assessment. However, there are no established methods to perform this critical task. Input parameters used for the prediction of seismic hazards, such as the time window of past data and effective prediction distance, are determined based on site-specific experience without statistical or physical reasons to support. Therefore, the accuracy of current seismic prediction methods is largely constrained, which can only be addressed by quantitively assessing the ST correlations of mining-induced seismicity. In this research, the ST correlation of seismic event energy collected from a study mine is quantitatively analysed using various statistical methods, including autocorrelation function (ACF), semivariogram and Moran's I analysis. In addition, based on the integrated ST correlation assessment, seismic events are further classified into seven clusters, so as to assess the correlations within individual clusters. The correlation of seismic events is found to be quantitatively assessable, and their correlations may vary throughout the mineral extraction process.

Progressive damage and fracture of biaxially-confined anisotropic coal under repeated impact loads

Coal masses in underground mines are in multiaxial stress states after excavation, and they are frequently subjected to dynamic loads particularly from continuous mining-induced seismic events. The changes of in-situ stress conditions and external dynamic loads may induce coal bursts, which involves the violent breaking of coal masses. Meanwhile, the coal bursting process may be influenced by bedding planes that are considered as unique structures in coal masses, which raises uncertainties of coal bursting mechanisms. This paper aims to reveal the progressive damage and burst proneness of anisotropic coal under coupled static stress and repeated dynamic loads. The tested coal specimens have five bedding plane orientations (β = 0°, 30°, 45°, 60°, and 90°), where β is the angle between the bedding plane and dynamic loading direction. A triaxial Hopkinson bar (Tri-HB) system is utilised to apply the biaxial static stress on the specimen and then followed by dynamic loading, and the stress and strain can be recorded by the system accordingly. After the loading test, ultrasonic wave velocity is measured to examine the degree of attenuation, and the quantification of microcrack characteristics is conducted by using the synchrotron-based X-ray computed tomography (CT). Then, the specimen is loaded again under the same biaxial static stress and dynamic load, and measured by ultrasonic sensors and X-ray CT. This process will be repeated until the final failure of the specimen. We found that both dynamic peak stress and P-wave velocity show a decreasing trend due to damage accumulation with increasing number of dynamic tests. The bearing capacities of anisotropic coal are mainly governed by bedding angles, for example, the specimen with β = 30° undergoes the lowest impact numbers, but the one with β = 90° can undergo the highest numbers. The crack initiation and propagation are progressively revealed by two-dimensional (2D) X-ray slices and 3D fracture network reconstructions. As impact number increases, both 2D and 3D fractal dimensions of cracks increase exponentially, whereas dynamic peak stress and P-wave velocity are negatively correlated with them.

What Governs the Spatial and Temporal Distribution of Aftershocks in Mining-Induced Seismicity: Insight into the Influence of Coseismic Static Stress Changes on Seismicity in Kiruna Mine, Sweden

ABSTRACTStrong mining-induced earthquakes are often followed by aftershocks, similar to natural earthquakes. Although the magnitudes of such in-mine aftershocks are not high, they may pose a threat to mining infrastructure, production, and primarily, people working underground. The existing post-earthquake mining procedures usually do not consider any aspects of the physics of the mainshock. This work aims to estimate the rate and distribution of aftershocks following mining-induced seismic events by applying the rate-and-state model of fault friction, which is commonly used in natural earthquake studies. It was found that both the pre-mainshock level of seismicity and the coseismic stress change following the mainshock rupture have strong effects on the aftershock sequence. For mining-induced seismicity, however, we need to additionally account for the constantly changing stress state caused by the ongoing exploitation. Here, we attempt to model the aftershock sequence, its rate, and distribution of two M≈2 events in iron ore Kiruna mine, Sweden. We could appropriately estimate the aftershock sequence for one of the events because both the modeled rate and distribution of aftershocks matched the observed activity; however, the model underestimated the rate of aftershocks for the other event. The results of modeling showed that aftershocks following mining events occur in the areas of pre-mainshock activity influenced by the positive coulomb stress changes, according to the model’s assumptions. However, we also noted that some additional process not incorporated in the rate-and-state model may influence the aftershock sequence. Nevertheless, this type of modeling is a good tool for evaluating the risk areas in mines following a strong seismic event.

An Experimental Investigation into the Strainburst Process Under Quasi-static Loading

Rockbursts cause damage to underground excavations in a sudden and violent manner and are associated with mining-induced seismic events. We propose a simple experimental method to study strainburst process in the laboratory, that simply involves a common compression testing apparatus and rock-coal-rock specimens. Strainbursts of coal samples are successfully produced and the burst process is monitored using high-speed camera and acoustic emission sensors. The strainburst mechanism is characterized by an initial ejection of small coal fragments followed by the ejection of large coal blocks. We found that the strainbursts are caused by the elastic strain energy stored in the rock samples during the uniaxial compression. The amount of the transferred energy is significantly less than the elastic energy stored in the coal sample but plays an important role in triggering strainbursts. The greater the transferred energy, the greater the damage severity of strainbursts occurred in the coal sample. Tensile cracking subparallel to the vertical loading direction and tangential compressive stress appears to play a dominant role in the strainburst failure mechanism.

Impact of High Energy Mining-Induced Seismic Shocks from Different Mining Activity Regions on a Multiple-Support Road Viaduct

In this paper, the dynamic responses of a large-scale multiple-support road viaduct to mining-induced seismic events registered in two regions of mining activity were compared. The regions differ in geological structure, which results in discrepancies in the dominant frequency content. Spatial variation of ground motion causing the kinematic excitation non-uniformity was accounted for in the dynamic analyses of this large-scale structure. Non-uniform mining-induced kinematic excitation models were proposed, with respect to the specificity of mining origin quakes. The dynamic performance of the viaduct was determined using three different methods of calculation: the time history analysis, the response spectrum analysis, and the multiple support response spectrum analysis. Both the uniform and non-uniform kinematic excitation models were adopted for the dynamic performance assessment. The research revealed that the dynamic response of some members of the structure, determined using the non-uniform excitation model, was significantly greater than that obtained for the uniform one. Hence, in the dynamic analysis of multiple-support structures under mining-induced events, the effect of spatial variation of ground motion should be considered. The study pointed out that the commonly used response spectrum analysis may lead to the underestimation of the dynamic response of large-scale multiple-support structures. Instead, the multiple support response spectrum method, which takes into account the non-uniformity of ground motion, is recommended as a conservative approximation. This method provides a safe upper estimation of the full-dynamic analysis results of large-scale structures under mining-induced tremors. Finally, the research indicated that the dynamic performance of a structure strongly depends on the frequency range attributed to a specific mining region. The dynamic performance of identical engineering structures under tremors of similar maximal amplitudes may differ significantly due to discrepancies in frequency contents of shocks occurring in various mining regions.

Prediction of Relatively High-Energy Seismic Events Using Spatial–Temporal Parametrisation of Mining-Induced Seismicity

Mining-induced seismicity has been reported almost in every mining country. Large seismic events with high-energy (HE) radiation can pose a serious threat to safe mining operations, which are the direct cause for rockbursts in underground mines. Over the last 30 years, statistical techniques to parameterise seismic data and predict seismic hazard have been developed significantly with promising results. However, similar to earthquake prediction, the prediction accuracy of HE seismic events remains a challenging task due to the complex nature of mining-induced seismic events. This paper aims to parameterise spatial, temporal and energy information conveyed by past seismic activities to provide early warnings for HE events. The 4D spatial–temporal seismic data were transferred to a 2D plane using principal component analysis (PCA) and then applied with a kernel density estimator to calculate their probability density function. The mode of probability density distribution, ρ(x)max, was proposed as a measure to quantify the clustering degree of past seismic events in the PCA space. The peaks of ρ(x)max were found as an effective precursor to predicting the onset time of future HE events. After inverse transformation, the spatial locations of ρ(x)max also indicate the potential high-risk areas that susceptible to HE events, which can be used to predict the onset location of HE events. As a supplement to the spatial–temporal information, energy/magnitude information was parameterised by the modified cumulative Benioff strain and b value. The accuracy of using these seismic parameters for the prediction of HE events has been assessed based on the seismic data collected from a Chinese coal mine.

Prediction of rotational ground motion for mining-induced seismicity – Case study from Upper Silesian Coal Basin, Poland

For decades, coal mining in the Upper Silesian Coal Basin (USCB) in Poland has been inducing annually more than 150 seismic events with local magnitude M > 2. Two sets of sensors were installed in the USCB in Poland to measure angular velocities and translational accelerations from seismic events induced by underground coal mining. In the period from December 2015 to November 2016, 503 seismic events were recorded with local magnitude M between 1.1 and 3.5. Analysis of measurement data for induced seismicity confirmed a strong linear correlation between peak ground acceleration of shear S-waves, PGA and peak ground motion of angular velocity, PRV. The observed correlation can be a result of previously observed coupling between the angular instantaneous velocity vector and the respective displacement vector at the ground surface. Three types of empirical equations for induced seismicity in Upper Silesia in Poland were developed to predict the peak rotational ground motion of the vertical (torsion), horizontal (rocking), and instantaneous 3D-xyz components. The prediction values of horizontal peak angular velocity ground motion indicate that for strong mining-induced seismic events in the USCB with a magnitude of M = 3.5, we can expect that the mean values of PRVH will reach 7.5 mrad/s in the epicentral area (r = 500 m) and with magnitude M = 4 respectively will reach 19.7 mrad/s. Such values of ground rotation may substantially contribute to the overall seismic response of civil infrastructure, particularly for tall buildings. The study of angular velocity ground motion is important not only from the cognitive point of view, but also from practical significance for assessing the seismic impact on very high buildings, chimneys and stability of slopes.

The effects of seismic anisotropy on mining seismology

SummaryMicroseismics – the detection and characterisation of mining-induced fracture events in the rockmass – is widely used to improve geotechnical understanding of the rockmass response to mining, and for hazard assessment of rockbursts and roof falls. These uses of microseismics require that the seismic events be accurately located, which can only be done using an accurate velocity model.In sedimentary environments, seismic velocity is typically anisotropic, usually being somewhat faster in a horizontal direction than in the vertical direction. However, anisotropy is seldom taken into account when processing microseismic data, resulting in locations of seismic events which can be significantly in error.This paper presents a technique for inverting a set of calibration shots with known location, along with a set of mining-induced seismic events, for an anisotropic velocity model. An example of the differences in event location is shown, illustrating the potential geotechnical significance.

Evaluation of seismic potential in a longwall mine with massive sandstone roof under deep overburden

A recent seismic event was recorded by a deep longwall mine in Virginia at 3.7 ML on the local magnitude scale and 3.4 MMS by the United States Geological Survey (USGS) in 2016. Further investigations by the National Institute for Occupational Safety and Health (NIOSH) and Coronado Coal researchers have shown that this event was associated with geological features that have also been associated with other, similar seismic events in Virginia. Detailed mapping and geological exploration in the mining area has made it possible to forecast possible locations for future seismic activity. In order to use the geology as a forecaster of mining-induced seismic events and their energy potential, two primary components are needed. The first component is a long history of recorded seismic events with accurately plotted locations. The second component is a high density of geologic data within the mining area. In this case, 181 events of 1.0 ML or greater were recorded by the mine’s seismic network between January, 2009, and October, 2016. Within the mining area, 897 geophysical logs, 224 core holes, and 1031 fiberscope holes were examined by mine geologists. From this information, it was found that overburden thickness, sandstone thickness, and sandstone quality contributed greatly to seismic locations. After the data was analyzed, a pattern became apparent indicating that the majority of seismic events occurred under specific conditions. Three forecast maps were created based on geology of previous seismic locations. The forecast maps have shown an accuracy of within 74%–89% when compared to the recorded 181 events that were 1.0 ML or greater when considering three major geological criteria of overburden thickness of 579.12 m or greater, 6.096–12.192 m of sandstone within 15.24 m of the Pocahontas number 3 seam, and a longwall caving height of 4.572 m or less.

Source parameters of seismic events potentially associated with damage in block 33/34 of the Kiirunavaara mine (Sweden)

Forty-six mining-induced seismic events with moment magnitude between −1.2 and 2.1 that possibly caused damage were studied. The events occurred between 2008 and 2013 at mining level 850–1350 m in the Kiirunavaara Mine (Sweden). Hypocenter locations were refined using from 6 to 130 sensors at distances of up to 1400 m. The source parameters of the events were re-estimated using spectral analysis with a standard Brune model (slope −2). The radiated energy for the studied events varied from 4.7 × 10−1 to 3.8 × 107 J, the source radii from 4 to 110 m, the apparent stress from 6.2 × 102 to 1.1 × 106 Pa, energy ratio (Es/Ep) from 1.2 to 126, and apparent volume from 1.8 × 103 to 1.1 × 107 m3. 90% of the events were located in the footwall, close to the ore contact. The events were classified as shear/fault slip (FS) or non-shear (NS) based on the Es/Ep ratio (>10 or <10). Out of 46 events 15 events were classified as NS located almost in the whole range between 840 and 1360 m, including many events below the production. The rest 31 FS events were concentrated mostly around the production levels and slightly below them. The relationships between some source parameters and seismic moment/moment magnitude showed dependence on the type of the source mechanism. The energy and the apparent stress were found to be three times larger for FS events than for NS events.

Non-stationarity and internal correlations of the occurrence process of mining-induced seismic events

A point process, e.g., the seismic process, is potentially predictable when it is non-stationary, internally correlated or both. In this paper, an analysis of the occurrence process of mining-induced seismic events from Rudna copper mine in Poland is presented. Stationarity and internal correlation are investigated in complete seismic time series and segmentally in subseries demonstrating relatively stable seismicity rates. It is shown that the complete seismic series are non-stationary; however, most of their shorter subseries become stationary. In the stationary subseries, the distribution of interevent time is closer to the exponential distribution, which is characteristic for the Poisson process. However, in most of these subseries, the differences between the interevent time and Poisson distributions are still significant, revealing correlations among seismic events.

OVERVIEW OF ROCK BURST RESEARCH IN CHINA AND ITS APPLICATION IN IVORY COAST

Rock bursts and mining-induced seismic events have serious socio-economic consequences for the Chinese mining industry, as its mines are extended to a specific depth. This paper mainly discusses deep mine hazards and rock burst problems in China. First, the definition of a 'rock burst' is given. The paper also introduces the current situation of deep mining and predicts its influences in Ivory Coast over the next ten years. Second, it explains the dynamic hazards arising from deep mining (the most dangerous hazard in deep mining in China).It then emphasizes that dynamic hazard control is a complex technology system that involve many techniques: evaluation, control, prevention and treatment. The burst technique system involves devices that are highly complex. Finally, the paper presents cases of controlled rock burst using the control technologies described, and considers their applications in Ivory Coast's deep mining industry.

Identifying local stress increase using a relative apparent stress ratio for populations of mining-induced seismic events

Large-magnitude mining-induced seismic events can occur as a result of local rock mass stress increase. High apparent stress events can be used as a proxy to identify increases in local rock mass stress conditions. This paper proposes a relative apparent stress ratio (ASR) for quantifying apparent stress of a seismic population. Variations in ASR are analysed over time with reference to seismic hazard inferred from the magnitude of seismic events. The proposed ratio is examined through examples of seismic populations from a single deep Canadian mine.

Analysis of post-blasting source mechanisms of mining-induced seismic events in Rudna copper mine, Poland

Abstract The exploitation of georesources by underground mining can be responsible for seismic activity in areas considered aseismic. Since strong seismic events are connected with rockburst hazard, it is a continuous requirement to reduce seismic risk. One of the most effective methods to do so is blasting in potentially hazardous mining panels. In this way, small to moderate tremors are provoked and stress accumulation is substantially reduced. In this paper we present an analysis of post-blasting events using Full Moment Tensor (MT) inversion at the Rudna mine, Poland, underground seismic network. In addition, we describe the problems we faced when analyzing seismic signals. Our studies show that focal mechanisms for events that occurred after blasts exhibit common features in the MT solution. The strong isotropic and small Double Couple (DC) component of the MT, indicate that these events were provoked by detonations. On the other hand, post-blasting MT is considerably different than the MT obtained for strong mining events. We believe that seismological analysis of provoked and unprovoked events can be a very useful tool in confirming the effectiveness of blasting in seismic hazard reduction in mining areas.

Clustering of mining-induced seismic events in equivalent dimension spaces

High energy release during seismic events induced by mining operation is one of the major dangers perturbing production in underground mines. In this work, temporal changes of seismic event parameters for one of the Rudna Mine (Poland) panels are investigated. The study aim was to find whether the temporal clustering of smaller events in different parameters can be observed before and after the high energy events (Ml ≥ 3) in the mining panel. The method chosen for analysis was the study of temporal variation of fractal dimension of the seismic events parameter sets composed from: the interevent epicentral distance (dr), logarithm of seismic energy (lE), and interevent energy coefficient (dlE), which is the absolute difference between logarithms of energy of two consecutive events. Temporal variations study was performed in equivalent dimension (ED) space. The transformation of the seismic source parameters into ED space allowed to estimate and compare the temporal changes of the fractal dimension of different parameter spaces using the same method—correlation fractal dimension, and then easily compare the obtained temporal changes of fractal dimension of different parameter sets. The effect of grouping is expressed by decrease of fractal dimension, which is connected with the similarity of events parameter values. The temporal changes of the fractal dimension of seismicity before the strong induced events would indicate some initiation phase of the process leading to the high energy release. In the case of the studied Rudna Mine panel, the temporal behavior of the fractal dimension values in different parameter spaces before seismic events showed significant changes before three out of four events with CLVD dominant source mechanisms.